1. Field of the Invention
This invention relates to a shift mechanism for an outboard motor.
2. Description of the Related Art
In outboard motors, an output of the internal combustion engine mounted thereon is transmitted to a forward gear or a reverse gear through a vertical shaft and is then transmitted to a propeller shaft. A shift is usually performed by moving a shift rod having a cam at its distal end in the lengthwise direction (vertical direction) to slide a shift slider in the horizontal direction such that a shifter clutch is switched from its neutral position to a forward position where it engages with the forward gear or a reverse position where it engages with the reverse gear.
Alternatively, the shift rod is provided with a rod pin at a position eccentric from the rod center axis in such a way that the rod pin is displaced to slide the shift slider such that the shifter clutch is engaged with the forward gear or the reverse gear to effect the shift.
The engagement of this shifter clutch and the forward/reverse gears is usually made by meshing projections formed on the shifter clutch with mating projections formed on the gears. Thus, most of the outboard motor shift mechanisms are usually constituted as a meshed type of clutch including the shifter clutch and forward/reverse gear projections to be meshed therewith, i.e., the so-called “dog clutch”. In this type of clutch, unless the rotational speed of drive shaft side (forward/reverse gears) and that of driven shaft side (propeller shaft that rotates integrally with the shifter clutch) are in synchronism with each other, projections formed thereon do not fit into mated recesses smoothly at the beginning of shift and an impact or shock may sometimes happen. If this happens, the outboard motor may vibrate and in addition, the drive train (including the projections, the vertical shaft, etc.) may have excessive stress.
In order to avoid this problem, it has been known to mitigate such an excessive stress by dividing the vertical shaft (drive shaft) into two shaft halves and by connecting them through an elastic member, as disclosed in Japanese Laid-Open Patent Application No. 2000-280983.
However, this also has disadvantages that it merely proposes mitigating the stress (that acts on the drive train) by the elastic member. In other words, since this technique does not aim to directly decrease the impact itself, it leaves much to be improved.
Aside from the above, when the shift rod is to be operated manually, since the operator tends to have an unpleasant operation “feel” owing to, for instance, heavy load, it has hitherto been proposed installing an actuator in the outboard motor and connecting it with the shift rod through a cable or a link mechanism to power-assist the driving of the shift rod, i.e. the shift, as taught in Japanese Patent No. 2817738.
The add-on system using such an actuator has disadvantages that the operation feel is degraded by plays in additional movable members in the complicated structure, that it makes maintenance tedious, and that it needs a space in the outboard motor.